Patent GB-1512707-A (GB)
Title: Semiconductor devices
IPC: H01L27/105
Filed: 1975-05-16
Abstract: 1512707 Semiconductor integrated circuit PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd 16 May 1975 [20 May 1974] 20841/75 Heading H1K A semiconductor integrated structure enabling an input signal, the amplitude of which varies stepwise or continuously to make connections between a common source of operating current and selected ones or groups of display devices comprises a semiconductor body with at least a surface region of one conductivity type, a plurality of first zones of opposite conductivity type each associated with a display device, a common electrode associated with a current source connected to one or more second surface zones of the opposite conductivity type and a gating structure to which the input signal is applied to provide a connection path of said opposite conductivity type between the common electrode and selected ones of groups of the first surface zones. Though the body itself may in certain structures constitute the gating structure this is usually an electrode on an insulating layer overlying the body surface which serves to control conduction through a surface zone of said opposite conductivity type extending between the first and second zones or preferably to generate on inversion layer the lateral extent of which is determined by the input signal. Such control of an inversion layer may be achieved with a gate electrode having a single connection if the threshold voltage beneath it is made to vary locally as by varying the surface doping of the semiconductor, the work function of the gate electrode material, the thickness or dielectric constant of the gate insulation or the amount of charge therein, e.g. introduced by ion implantation or trapped at the interface between layers of different composition. It is preferably achieved by using a resistive gate electrode with spaced contacts to provide spatial variation in the gate voltage for a given input signal. A structure with such a gate electrode is shown in Fig. 1 where the supply voltage for the display devices 35-39, which may be incandescent lamps, discharge tubes, liquid crystals or LEDs, e.g. integrated into the same semiconductor body, is designated 140. Control voltages 33, 34 are adjusted so that with zero input from signal source 1 no inversion layer is present to connect P zone 17 to any of P zones 6-10. As the input voltage increases the inversion layer expands from left to right to reach zones 6-10 in turn to switch on an increasing number of display devices. In a variant in which the zone 17 is replaced by an interconnected set of zones each opposite to one of zones 6-10 the parts of the common gate electrode of polycrystalline silicon between the pairs of zones is heavily doped and thus rendered conductive when used as masking during diffusion of the zones, thus improving the sharpness of commutation. Such an improvement is also achieved in the Fig. 6 structure (not shown) because the inversion layer expands parallel rather than normal to the paths between the zones 63 connected to the common electrode and those associated with the display elements. In addition the controlling parts 70 of the gate electrodes underlying the resistive strip 71 may be conductive. The structure is defined in a P-type island on an N substrate which may include an analogue amplifier for the input signal and flip-flop amplifiers connected between output conductors 67 and the display devices. In a development of this structure enabling individual display devices to be selectively connected, shown in Fig. 8, zones 66 (indentifiable in Fig. 8 as 87) are connected to the display devices via further structures provided with a second common resistive gate 89 having a reverse voltage gradient, to which the input signal is applied in antiphase, the arrangement being such that one (or a fixed number of) pairs of zones 81 and 84 are connected for a given input signal. If gates 88 and 89 are suitable overlapped the zones 87 may be dispensed with. The output diffusions 84 are mutually isolated by channel stoppers provided at 99 by local thickening of the insulation or heavy doping of the surface. Alternatively the entire surface outside the gating regions is heavily doped. In a variant of this structure (Fig. 9, not shown) in which the common electrode is connected to each output diffusion via a parallel connected pair of inversion layers controlled by gates 88, 89 respectively rather than a series connected pair as in Fig. 8 the supply voltages are adjusted so that for any given input voltage only one (or a fixed number) of output zones are not connected. In all cases the resistive gates may be replaced by a set of gates connected to tappings on a resistor, e.g. of diffused type. Also the manner in which the display changes with input voltage may be altered by varying the doping or width of the resistive gate along its length or the spacing along it of the portions controlling the inversion layer. Some manufacturing details are given. Switches, e.g. integrated IGFETs with a common gate may be disposed between the output diffusions and the display devices to black reading of the display while the input is being changed.